Mainly used solar cells nowadays are silicon crystalline solar cells using silicon substrates having PN junctions on the silicon substrates. Meanwhile, compound semiconductor solar cells using direct bandgap compound semiconductors with large optical absorption coefficients provide for higher photoelectric conversion efficiency than crystalline silicon solar cells. Many of the compound semiconductor solar cells currently under development are multijunction compound semiconductor solar cells with a multijunction (tandem) structure having a plurality of photoelectric conversion layers (PN junction layers) with bandgaps that are different from one another. Having ability of effectively using solar light spectra, this kind of solar cells allows for higher photoelectric conversion efficiency than single junction compound semiconductor solar cells with single photoelectric conversion layer.
Currently under active study for multijunction compound semiconductor solar cells having a plurality of photoelectric conversion layers with bandgaps different from one another is a system with which lattice match by means of epitaxial growth is taken into consideration, i.e., a lattice-matched system. With respect to the lattice-matched system, a multijunction compound semiconductor solar cell with three photoelectric conversion layers has been developed which includes an InGaP photoelectric conversion layer/a GaAs photoelectric conversion layer/a Ge photoelectric conversion layer from the side on which solar light is to be incident (a light receiving surface side). The bandgap of the InGaP photoelectric conversion layer is about 1.87 eV, the bandgap of the GaAs photoelectric conversion layer is about 1.42 eV, and the bandgap of the Ge photoelectric conversion layer is about 0.67 eV.
Outline cross-sectional views of multijunction compound semiconductor solar cells of related art are depicted in FIGS. 16(a) and 16(b). The multijunction compound semiconductor solar cell depicted in FIG. 16(a) has a top cell 501, a middle cell 502, and a bottom cell 504 that are arranged in this order from the solar light incident side. Top cell 501 on the light receiving surface side has on a surface thereof a first electrode 505, and bottom cell 504 on the side opposite the light receiving surface side (i.e., a back surface side) has on a back surface thereof a second electrode 506. The photoelectric conversion layer of top cell 501 has the largest bandgap, the photoelectric conversion layer of middle cell 502 has the second largest bandgap, and the photoelectric conversion layer of bottom cell 504 has the smallest bandgap.
As depicted in FIG. 16(a), solar light enters from the side of top cell 501 and propagates toward bottom cell 504. During the propagation, the photoelectric conversion layers of top cell 501, middle cell 502, and bottom cell 504 allow wavelengths of the solar light to be absorbed therein based on their respective bandgaps, so as to effect conversion (photoelectric conversion) into electric energy. It is to be noted here that top cell 501, middle cell 502, and bottom cell 504 each comprise a plurality of semiconductor layers including one photoelectric conversion layer.
To effectively utilize solar light spectra in the triple-junction multijunction compound semiconductor solar cell of top cell 501/middle cell 502/bottom cell 504 as depicted in FIG. 16(a), it is considered that a desirable material combination is such that the respective photoelectric conversion layers of the cells have bandgaps of 1.93 eV/1.42 eV/1.05 eV from the light receiving surface side. In order to obtain a multijunction compound semiconductor solar cell with higher photoelectric conversion efficiency, considered as a material for bottom cell 504 is a material that allows the photoelectric conversion layer of bottom cell 504 to have a bandgap on the order of 0.9 to 1.1 eV.
InGaAs is proposed as one of the materials that have a bandgap on the order of 1 eV. In the case where InGaAs is used as a material of bottom cell 504, InGaP is used for top cell 501, and GaAs is used as a material of middle cell 502 to fabricate a multijunction compound semiconductor solar cell, GaAs constituting middle cell 502 is different in lattice constant from InGaAs constituting bottom cell 504, and the difference in lattice constant is as large as about 2%. Thus, as depicted in FIG. 16(b), a multijunction compound semiconductor solar cell is under development which has a buffer layer 503 with varied lattice constants disposed between middle cell 502 and bottom cell 504.
Non-patent Literature 1 (J. F. Geisz, et al., “Inverted GaInP/(In)GaAs/InGaAs triple-junction solar cells with low-stress metamorphic bottom junction”, 33rd IEEE Photovoltaic Specialists Conference San Diego, Calif., May 11-16, 2008) discloses a multijunction compound semiconductor solar cell including InGaP (the top cell)/GaAs (the middle cell)/InGaAs (the bottom cell), wherein top cell 501 (InGaP) and middle cell 502 (GaAs) are lattice-matched, while a buffer layer 503 having varied InGaP lattice constants is disposed between middle cell 502 (GaAs) and bottom cell 504 (InGaAs) that are different in lattice constant.
FIG. 17(a) depicts a relationship between the lattice constant and the film thickness of the related art where a multijunction compound semiconductor solar cell is formed on a semiconductor substrate 507 (a GaAs substrate) as depicted in the outline cross-sectional view of FIG. 17(b). FIG. 18(a) depicts a relationship between the lattice constant and the film thickness of the related art where a multijunction compound semiconductor solar cell is formed on a semiconductor substrate 507 (a GaAs substrate) as depicted in the outline cross-sectional view of FIG. 18(b). The multijunction compound semiconductor solar cells on the semiconductor substrates 507 (the GaAs substrates) that are depicted in FIGS. 17(b) and 18(b) are fabricated in the following manner. In FIGS. 17 and 18, illustration is not given of tunnel junction.
First, InGaP crystal that is lattice-matched with the GaAs crystal constituting a semiconductor substrate 507 is epitaxially grown on semiconductor substrate 507 (the GaAs substrate), so as to form a top cell 501, and then GaAs crystal that is lattice-matched with the InGaP crystal constituting top cell 501 is epitaxially grown, so as to form a middle cell 502.
Next, InGaP crystal is epitaxially grown to form a buffer layer 503A in such a manner that the lattice constants thereof increase at equidistances (the lattice constants increment by a constant amount of increase). Subsequently, InGaAs crystal is epitaxially grown to form a bottom cell 504, such that a multijunction compound semiconductor solar cell wafer is fabricated.
As depicted in FIG. 17(a), in the multijunction compound semiconductor solar cell wafer depicted in FIG. 17(b), the lattice constants of the InGaP crystal of buffer layer 503A, which crystal being adjacent bottom cell 504, is larger than the lattice constant of bottom cell 504.
Further, as depicted in FIG. 18(a), in the formation of the multijunction compound semiconductor solar cell on semiconductor substrate 507 (the GaAs substrate) that is depicted in FIG. 18(b), the lattice constant of InGaP crystal of buffer layer 503B, which crystal being adjacent bottom cell 504, is smaller than the lattice constant of bottom cell 504. Similar description is found in Patent Literature 1 (Japanese Patent Laying-Open No. 2007-324563).
FIGS. 17(b) and 18(b) exemplarily depict a method referred to as an invert fabrication process wherein cells are stacked on semiconductor substrate 507 in order from the cell disposed on the light receiving surface side of the multijunction compound semiconductor solar cell, namely, in the order of top cell 501, middle cell 502, and bottom cell 504, and such a laminate structure is referred to as an “inverted triple junction”.